How do I increase the mouse manipulation speed?
While you're doing the inset (or most any transform), you can actually just start typing a number and Blender will use that and ignore your mouse pointer. You can see it in the header at the bottom of the view.

With Inset specifically, the scale appears to be that 1.0 insets all the way to a point at the center of the face, 0.5 is halfway, etc.